An immature permanent tooth can be defined as a newly erupted permanent tooth with incomplete root apex formation. Once erupted, root development and apical closure take approximately 3 years[1]. Root development involves interactions between the dental follicle, epithelial root sheath of Hertwig, and dental papilla. Hertwig’s epithelial root sheath is responsible for the future shape of the roots[2].

Despite studies on dental eruption timing for children worldwide[3-5], fewer studies have examined dental development as a process extending across years or decades. The existing literature primarily focuses on the mandibular teeth[6-9]. In addition, while Moorrees et al.[10], Haavikko[11], and Liversidge[12] have published papers on tooth formation, their data are quite outdated. Among the early staging techniques, the method by Demirjian et al. is still widely used[13]. Many researchers have applied Demirjian’s technique to children of different countries[14,15]; however, few studies have examined tooth root development beyond its use in obtaining dental age.

Despite its importance, research on root development and apical foramen of permanent maxillary central incisors is limited. These incisors, which are located prominently at the center of the face, typically erupt between the ages of 7 and 8 years during the transition from deciduous to mixed dentition[3]. Immature teeth are susceptible to pulp involvement because of factors such as trauma, caries, or iatrogenic causes. Dental trauma is a prevalent reason for children with immature permanent teeth to undergo root canal treatment or pulpal therapy, affecting approximately 20% of children and adolescents with permanent dentition[16]. Studies have indicated that 13.5% of children experience traumatic injuries to their permanent teeth before the age of 12[17]. Among these, the maxillary central incisors are most frequently affected.

These teeth must be preserved to ensure their long-term retention. The appropriate type of pulp therapy is determined by whether the pulp is vital or nonvital. After diagnosis, several treatment options are available for vital teeth, including pulp capping and pulpotomies. These treatments aim to preserve the health of the root pulp, support spontaneous pulp growth, and require continuous monitoring until root formation is complete. The speed of root growth and apical foramen closure rate are vital in determining treatment duration[18]. However, current research lacks comprehensive data on these parameters for maxillary central incisors. The duration of root growth, periods of rapid root elongation, timing of apical foramen closure, and degree of apical closure required for effective endodontic treatment are less understood.

This study aimed to evaluate the growth velocity of root elongation and the timing of apical foramen closure in maxillary permanent central incisors by analyzing a series of panoramic radiographs from a cohort of Korean children, measuring root length and apical foramen width over time. Specifically, the focus is on the development of maxillary central incisors, given their critical role in early dental development and treatment, particularly in cases of trauma or other dental issues. Because panoramic radiographs are essential in dental examinations, they can be incredibly useful for assessing the developmental progress of permanent teeth. Ultimately, the timing of the eruption of these teeth may be predicted with greater precision, an ability that would have important clinical implications[19]. A better understanding of the variability in dental growth and development would provide greater insights into human adaptation and evolution of hominid growth and development[20,21]. Studies of human dental development, in addition to their clinical utility, contribute significantly to this theoretical enterprise. As our predictive ability improves, so does our ability to understand previous developmental patterns.

For theoretical and practical reasons, gaining a more nuanced understanding of dental maturation is essential[15,20]. Comprehensive datasets from large longitudinal studies conducted in the mid-20th century remain invaluable, and smaller-scale studies focusing on contemporary pediatric populations are crucial for addressing specific questions and providing updates that account for potential secular trends. These studies also help in examining regional variations in dental maturation.

In the present study, the percentage curves generated from the age in months (Fig. 2 and Fig. 3) exhibit a simple pattern of root elongation through time, relative to the total tooth length, for the maxillary central incisors between the ages of 4 and 13. Because for the ages surveyed the crowns of these teeth have completed formation and thus should have a stable length, the pattern displayed implies the velocity of the root growth over this age span, which increases until the peak velocity, and decreases afterward. In the study of Moorrees et al.[10], root formation, again expressed as a mean time interval from crown formation to root complete, occurred in 3.3 years for the maxillary central incisors. The root complete was around the average age of 8.5 years, with 2 standard deviations at approximately 7.7 - 11.17 years[10]. In the present study, the average age of the last panoramic radiograph was 11.05 years, which is approximately 3 years later than the result by Moorrees et al.[10]. For accurate interpretation and application, terminologies should not be different between the two studies. In the study by Moorrees et al.[10], the term “root complete” was defined as “root length complete,” indicating the full-length extension of the root. After “root complete,” a distinct term of “apex closed” refers to the apex closure of the root. However, in the present study, age data of “apex closed” was only collected; thus, a precise comparison cannot be made between the results of the two studies. However, the age in the present study is also not the average age of apical closure because it is the point when the apical closure has already happened, not the exact point of apical closure; thus, in reality, the age at apical closure must be earlier than 11.05 years. To compare with the results of Logan and Kronfeld[1], the age of enamel formation completion of the maxillary central incisors is 4 - 5 years, and the age of eruption is 7 - 8 years. The age of complete root formation and apical foramen closure is reported as 10 years, which is pretty much consistent with the results of the present study because the age at alveolar eruption was 7.18 ± 0.65 years, and the age at complete root formation and apical foramen closure was expected to be slightly earlier than 11.05 years. Haavikko[11] reported the median of 9.3 years as the age for apical closure for the maxillary central incisors for females and 9.8-year age for males. This was also earlier than the mean age calculated from the present study.

Moorrees et al.[10] implied that root growth velocity progressively increased from root one - quarter to root complete. The present results are more consistent with those of Haavikko[11]. She proposed a pattern similar to that found in the present study, that is, that tooth roots had growth spurts, usually between root one-half and root complete; for incisors and canines, the spurt was suggested to occur earlier, in the middle two-quarters of root development. The “spurt” for the maxillary central incisors for females was at about the root one-quarter.

The present study indicated the equation of the trend line of the root length and apical foramen width in both sexes. By substituting the age in months into the equation representing the trend line, the root length or apical foramen size can be calculated. Conversely, by entering the desired root length or apical foramen size into the equation, the corresponding age in months at which that length or width occurs can be determined.

Typically, females are more advanced in their relative root growth timing than males. Although the root growth timing was earlier in females than males in their ages, this study also showed that the maximum root growth rate, which is the slope of the trend line of the graph in Fig. 3, was faster in males than in females. Males tended to show faster root growth at the growth spurt and tended to prolong the growth until later than females. Nevertheless, the timing and rate of apical foramen closure appeared to be faster in females. The timing of alveolar eruption also showed sex difference, and females tended to show earlier eruption than males, although the difference was not significant. These findings are consistent with those presented by Demirjian et al.[13], indicating that overall tooth growth timing is generally more advanced in females than in males[19].

Because other previous studies lack data on the root development of maxillary central incisors, direct comparisons with previous studies are difficult. However, because several studies were performed with other teeth, it could be compared if some similarities or differences were present between these teeth. Smith and Buschang[7] examined the tooth root development of mandibular canines and premolars on an orthodontic sample. The study examined the velocity at milestones of root one-half and root three-quarters. At root one-half for the mandibular canine root growth, the velocity slightly increased. The velocity was nearly stable at root three-quarters but was past the peak velocity. For the mandibular first premolar, velocity was around the minimum at root one-half and was nearing peak velocity at root three-quarters. For mandibular second premolars, velocity was already increasing at root one-half and was near the peak velocity at root three-quarters. In the present study, at approximately root one-quarter, the velocity started to increase. Root one-half was around the peak velocity of root growth. From the point around root three-quarters, the graph started to show a plateau, which means that the root growth was nearly completed. Liversidge et al.[15] reported mean ages for stage H (apex complete) were 7.90 ± 0.11 years for females and 8.34 ± 0.11 years for males for the mandibular central incisors. Because the study of Liversidge et al.[15] and the present study both used intermittent data, which is the radiograph of certain timing, it is not accurately the timing of the apex complete.

According to Simpson and Kunos[8], the rate of morphological change of the mandibular central incisors increases after the root is approximately 15% complete. The rate of root elongation remains high until approximately 90% of the root is completed. As expected, the rate of root elongation is the greatest in the period accompanying emergence[22]. This pattern is slightly different from that in our study, in which at 30.8% of root completion, the root length velocity started to accelerate. After 85% - 90%, the graph tended to show a plateau. Similarly, while Simpson and Kunos[8] found the mandibular central incisors to reach their full lengths between 7 - 8 and 1/4 years, our sample shows some continued slow growth beyond these ages, particularly for males. Although the general pattern showed similarity, this might be the difference between the pattern of maxillary and mandibular incisors. Maxillary central incisors tended to accelerate later than the mandibular central incisors in their root length growth.

Studies have compared modern humans with past ancestors. Dean and Vesey[23] found a clearer pattern of the peak extension rates for the posterior teeth (molars) than for the anterior teeth (incisors and canines). In the study by Dean and Vesey[23], as part of anthropological research, the root growth rates at the time of eruption in modern humans and great apes were compared through histological analysis. This study used the circadian incremental markings in dentin. De Castro et al.[20] compared the perikymata of Homo sapiens and Homo heidelbergensis to investigate crown formation rates. This study demonstrated that ancient hominins completed crown formation at a shorter time than modern humans.

Previous studies have also investigated the relationship between the timing of eruption and root growth. In the study of Feasby[9], the root growth rates of mandibular canines to second molars were measured using lateral cephalograms. Eruption rates were also measured using the inclination from the mandibular inferior border to examine its correlation with root length. Of particular interest was the low correlation between the maximum eruption rate and increments in root growth. This finding indicates that the eruption surge and root increments were independent variables. In addition, when the mean maximum eruption rates were compared with the mean root growth during the eruption, the eruption exceeded root growth by as much as 4.6 mm. These circumstances indicate that the most dramatic phase of the eruption could not be accounted for by root growth alone. In comparison, the present study measured the root length and age at the time of eruption; however, the eruption rate was not of interest. This can be investigated in further studies, and as the previous studies have suggested the line of the mandibular inferior border as the criteria to measure the eruption of mandibular teeth, a new line of criteria for the maxillary teeth must be presented.

When measuring the Demirjian stage and eruption, stage F was the most significant eruption, followed by stages E and G. Demirjian stage F occurs when the walls of the pulp chamber form an isosceles triangle. The apex ends in a funnel shape. The root length is equal to or greater than the crown height[13]. When stage G is past and if the maxillary central incisor is still not erupted, then proactive treatment for eruption management should be needed. Given that Demirjian’s technique has been shown to have varying accuracy across ethnicities, the findings in this study of Korean children should be considered within the context of ethnic differences in dental development[24]. According to the study done by Jeong et al.[25], both the root length and tooth volume of the maxillary incisors with eruption disturbances were significantly smaller than those of normally erupting incisors. This suggests that eruption disturbances may influence tooth development, underscoring the need for appropriate interventions.

This study found a high correlation between root length and apical foramen width. Both the elongation of the root and constriction of the apical foramen would be naturally associated because they are both growth processes. Thus, if the constriction of the apical foramen has already occurred in teeth with shorter than normal root lengths, further root growth may be difficult to expect.

The limitations of this study include the potential distortion from using panoramic radiographs to measure the root length and apical foramen width, and only the mesiodistal width of the apical foramen was measured. The panoramic radiographs were also not taken at consistent intervals. Future studies conducted on various teeth could provide more valuable information regarding human dental development.

This study provides information on the developmental patterns of root growth and apical foramen closure in maxillary permanent central incisors. The findings reveal the critical periods of rapid root elongation and apical foramen closure, which are essential to understanding the timing and management of dental treatments, particularly in pediatric patients. While the results suggest that females generally tend to experience earlier and faster apical foramen closure than males, this observation reflects a trend rather than a statistically significant difference. Additionally, significant correlations were found between root length and apical foramen width across both sexes. Further studies using a similar methodology could explore maxillary lateral incisors, canines, and premolars to offer additional insights into the developmental patterns of these teeth.